Various embodiments of the present disclosure new (co)polymeric materials suitable for use as photoalignment layers. The new photoalignment materials comprise a photo-orientable structurally anisotropic polymer network that displays improved adhesion to substrate surfaces and can align thicker monomeric and polymeric liquid crystal layers. Methods of making and applying the new photoalignment materials are also disclosed.
Liquid crystal materials are used in a variety of applications where the liquid crystal material is deposited as a layer on the surface of a substrate. The successful functioning of a liquid crystal device depends, at least in part, on the ability of the liquid crystal molecules within the layer to adopt and maintain a particular alignment or orientation. These liquid crystal layers may be aligned or oriented using various methods. One approach is to coat the surface of the substrate with an orienting layer prior to the application of the liquid crystal layer. The orienting layer may then be used to orient the liquid crystal material on the substrate, for example, by rubbing or irradiation with polarized electromagnetic radiation. The orientation layer defines the direction of orientation of the liquid crystal molecules of the layer with the result that the longitudinal axes of the molecules become aligned with the direction of orientation defined by the orientation layer. In addition to directional alignment, the orientation layer may also impart an angle of tilt to the liquid crystal molecules, so that the molecules align themselves at an angle to the surface of the orientation layer rather than lying parallel to the surface.
Orientation of polymer layers by irradiation with polarized electromagnetic radiation has been known. Irradiation based orientation overcomes certain drawbacks associated with orientation by uniaxial rubbing, such as, for example, dust generation, heat generation, destruction of thin films, and lack of structuring capability. Further, orientation by irradiation also allows for the possibility to provide distinct areas having different orientation relative to neighboring areas. Examples of photo-orientable alignment materials include polymer-bonded photoactive cinnamic acid derivatives, coumarin derivatives, cis/trans isomerizable azo derivatives, and photochemically decomposable polyimide derivatives.
However, even with photoalignment layers with photochemically active alignment chromophores certain problems exist. Typical prior art alignment layer materials used in liquid crystal devices demonstrate poor adhesion, and are produced using high processing temperatures (200° C. to 250° C.) that are not compatible for certain substrates. For example, for certain applications good adhesion to the substrate is necessary. In addition, adhesion between layers in applications where a liquid crystal layer or other layer is deposited on the surface of the photoalignment layer is also necessary. In applications where adhesion levels are not sufficient, peeling of the photoalignment layer from the substrate surface and/or peeling of subsequent layers from the surface of the photoalignment layer may be observed.
In addition, certain applications, such as ophthalmic applications, utilize liquid crystal layers of greater than 20 microns in thickness. In these applications, photoalignment layers that are capable of aligning adherent liquid crystal layers of up to 1,000 microns in thickness are desired. Thus, photoalignment materials that may be used to form layers having improved adhesion properties and thicker aligned liquid crystal layers, relative to known photoalignment materials, are desired.